In the case where a treatment for restoring cast crown or a treatment for defect prosthesis is required to restore a tooth, first, a cast of an abutment tooth is taken. Next, the taken cast is used to prepare a model made of plaster. Then, a material which substitutes for a damaged portion of the tooth is produced based on the model, and the resultant material is attached to the abutment tooth. The cast of the abutment tooth is referred to as impression, and a cured product to be used for taking the impression is referred to as impression material.
In general, an alginate impression material, agar impression material, silicone gum impression material, polysulfide gum impression material, or polyether gum impression material is used as an impression material. Of those, the alginate impression material is inexpensive and easy to handle, and hence the material is most widely used. The alginate impression material includes a base material containing mainly an alginate and a curing material containing mainly calcium sulfate. When the base material and curing material are kneaded in the presence of water, a gel-like cured product is obtained.
A process for taking an impression using an alginate impression material is performed according to the following procedures. First, a product prepared by kneading a base material and a curing material before curing is placed on an impression tray which is made to be similar shape of arrangement of teeth. Next, the tray where the impression material is placed is pressed against the teeth so that the tray covers the teeth in the oral cavity. Then, after the impression material is cured, an integrated product of the impression material and tray is removed from the teeth and taken out from the oral cavity.
Trays to be used for taking impressions are broadly divided into two types: ready-made trays and custom-made trays. The ready-made trays are ones which have predetermined sizes and shapes. Specific examples of the ready-made trays include metallic trays such as stainless, brass, and chrome-plated brass trays. Meanwhile, the custom-made trays are ones which have shapes suitable for individuals. Specific examples of the custom-made trays include resin trays which include a polymethacrylate and modeling compound trays which include a thermoplastic resin.
The alginate impression material has low adhesion property to the above-mentioned trays. Therefore, when the impression material is removed from teeth, the material may peel off from the tray. Peeling of the impression material from the tray may cause a significant change in the shape of the impression, so there occurs a problem that an impression with high accuracy cannot be obtained.
To solve the above-mentioned problems, net-like trays, undercut-like trays, or trays having punch holes may be used. When a tray having one of those shapes is used, the holding force between the impression material and tray is improved because the area of the impression material which contacts with the tray increases. Therefore, it is possible to prevent the impression material from peeling off from the tray.
On the other hand, in the case of using trays other than trays having the above-mentioned shapes, i.e., in the case of using, for example, ready-made or custom-made trays having plate-like shapes, it is necessary to enhance the holding force between the impression material and tray by another method. As one of the methods, a method for adhesion of a tray to an impression material using an adhesive agent containing an alkaline metal earth has been suggested (see Patent Document 1, for example).    Patent Document 1: JP 10-175812 A